Corbelled arch in the Caracol, observatory at Chichen Itza

The Mayans had very advanced technology by American standards, but there are two crucial inventions that they never made: the wheel, and the arch.

Since they never developed the arch, they could never build large enclosed halls in their stonework. In lieu of proper curved arches, they used the "corbelled arch", which is basically two walls slanting toward one another until they touch.